The present invention relates to a new and improved device for selectively determining the components of gas mixtures by means of a gas sensor containing a predetermined number of sensor elements which change their electric conductivity under the action of the components of the mixture to be determined and further containing an evaluation device or circuit arrangement.
A gas sensor of this type has been presented at the 32nd Electronic Components Conference which was held at San Diego, Calif., from May 10 to May 12, 1982.
Gas sensors have been used for some time for different purposes like, for example, environmental protection, garage monitoring, fire protection and explosion protection. For this purpose inexpensive sensors are utilized which are made of metal oxides having an electric conductivity which depends on the concentration of the gases to be detected in the ambient air. The concentration of the gas to be detected can be derived from the variation in the electric conductivity. In order to carry out the gas detection with such sensors the sensor must be brought to a temperature of about 450.degree. C. The known gas sensors are made of a finely ground metal oxide powder sintered to a carrier or substrate which is provided with electrodes. Such gas sensors exhibit only very little selectivity in their gas detection properties. The measured values obtained by using these gas sensors do not permit any conclusion with respect to which component in a gas mixture has been detected.
The individual components of gas mixtures cause widely different changes in the conductivity of the gas sensors, i.e. the gas sensors have different sensitivities with respect to the components of the gas mixture. It is therefore desirable to obtain data about the kind of gas to which the gas sensor responds. It is important, for example, for the supply of city gas or natural gas to residential areas that the exit of gas at leakage locations and toxic carbon monoxide which is formed by incomplete combustion processes are rapidly and reliably detected. Ethanol vapor represents a frequently interfering gas in such cases and frequently occurs, for to the effect that the gas sensors which are sensitive to carbon monoxide and hydrocarbons also respond to ethanol vapor.
Therefore, at the initially mentioned 32nd Electronic Component Conference a gas sensor has been suggested in which three gas sensors are provided which consist of different constituent materials. Two of the gas sensors serve for the detection of methane which is the main component in natural gas and for the detection of hydrogen and/or carbon monoxide which are components of city gas or coal or are formed by incomplete combustion. Since the two sensor elements of the two gas sensors also respond to ethanol vapor which represents the interfering gas, the concentration of ethanol vapor is measured by means of a third gas sensor which specifically reacts to ethanol vapor. The measured value determined by the third gas sensor is accounted for in determining the concentrations of methane and carbon monoxide by means of the other two sensor elements or gas sensors. Such gas detector, however, has the disadvantage that it responds only to two components of gas mixtures while a further specific component can be excluded as an interfering factor.